The fare conditions depend on your point of origin and it is hard to make general statements. Could be that I7SALE have specific conditions in your case.

But as long as there I availability from your point of origin, the fare is usually repriced on your historical fare. That means that you have to pay the change fee and any change in taxes and carrier surcharges, that's all. I have done it many times on discount I fares.

BTW, there is nothing as "regular I fare" as there can exist many I**** fares at the same time.